The invention relates to a method and ballast for operating a lamp fitted with a fluorescent tube.
E 0 889 675 A1 discloses two ballasts with which several different types of fluorescent tubes can be operated under optimised operating conditions. However, the degree of optmisation of the operating conditions of a fluorescent tube that can be achieved with these ballasts leaves much to be desired.
One of the ballasts pursuant to EP 0 889 675 A1 provides for a preheating phase of the electrodes. It is a ballast for hot-start fluorescent tubes. With a ballast of this kind, the electrodes projecting into the interior of the tube at the ends of the discharge tube of the fluorescent tube are preheated. The electrodes, which are provided with an emitter material, emit ions in this process, causing the gas contained in the discharge tube to become electrically conductive. Only after this preheating phase is the so-called discharge path of the fluorescent tube ignited. This procedure spares the electrodes. The electrode resistance of the fluorescent tube that occurs is measured during the preheating phase, in order to draw an indirect conclusion as to the electrode temperature and perform gentle preheating.
Moreover, according to EP 0 889 675 A1, conclusions as to he type of fluorescent tube are said to be drawn with the aid of the electrode resistance measured. The fluorescent tube is initially supplied with a low current and the electrode temperature is measured indirectly. If the initial current is insufficient to heat the electrodes to an expected temperature, the current is increased in steps, until the electrode resistance or the expected electrode temperature is reached. However, as there are different types of lamps that have the same or very similar electrode resistance values at operating temperature, the electrode resistance is not a definitive criterion for distinguishing between lamp types. When using the known ballast, optimum operating conditions can only be set if the lamp types to be recognised have substantially different electrode resistances.
In another ballast known from EP 0 889 675 A1, the simple method of measuring the lamp voltage is applied in order to determine the type of fluorescent tube, rather than the more complex measurement of the electrode resistance. As the indirect measurement of the temperature via the electrode resistance is eliminated, this design falls back on the sample method of measuring the lamp voltage. Owing to the absence of optimised preheating, the electrodes of the fluorescent tube are subject to elevated wear when using this ballast. Furthermore, unequivocal distinction between different lamp types is again impossible if they have identical or very similar lamp voltages. Likewise, optimum operating conditions can only be set with this ballast if the lamp types to be recognised have markedly different lamp voltages.
The object of the invention is therefore to propose a simple method, and a ballast for implementing the method, by means of which a large number of commercially available types of fluorescent tubes can be operated with a higher degree of optimisation.
According to the invention, the object is solved by a method for operating a lamp fitted with a fluorescent tube, where the operating data of certain recognisable lamp types, i.e. at least the rated lamp voltage, the rated lamp current and the preheating currents and preheating times for preheating the electrodes are stored in a register, where the preheating currents are allocated to predefined electrode resistance ranges, the electrode resistance is measured during a preheating phase, the preheating current allocated to the measured electrode resistance and the allocated preheating time are set, the fluorescent tube is operated with a dimming current of known intensity for a predetermined time during a starting phase following on from the preheating phase, the prevailing lamp voltage of the fluorescent tube is measured after the starting phase, the register is then searched for the rated lamp voltage that comes closest to the measured lamp voltage of the fluorescent tube and the operating data required for operation of the fluorescent tube and allocated to the measured lamp voltage by the register are then set.
The recognition of the lamp type in the present invention is based on the principle of lamp voltage measurement during the starting phase of the fluorescent tube. However, the electrode resistance is also known, having been determined during the preceding preheating phase, thus providing another selection criterion for accurate determination of the lamp type. Thus, the method according to the invention not only provides electrode-sparing hot-starting of the fluorescent tube, but also permits accurate determination of the lamp type.
The term xe2x80x9coperating dataxe2x80x9d is not taken only to mean the parameters required directly for operation of the fluorescent tube. It is also possible to store operating data, such as maximum lamp voltages and currents, electrode resistances and temperatures, that occur under abnormal operating conditions, for instance in order to bring about a safety shutdown, if appropriate.
To be able to understand the invention, it must be pointed out that the register can be used to store operating data, such as the rated lamp voltage and the rated lamp current, in direct form or, alternatively, in the form of other values that are linked to the operating data by way of correlation.
The invention also includes the option of altering and setting operating data of the fluorescent tube directly or of setting them indirectly via variables naturally linked to them. For example, the dimming current and the lamp current can be set by altering the frequency of the alternating current applied to the fluorescent tube in operation.
The term xe2x80x9cshort-termxe2x80x9d operation is intended to mean a predefined operating period amounting to between a few seconds and several minutes.
The dimming current set at the beginning of the starting phase is equal to the lowest rated lamp current stored in the register, or is greater than this. If it corresponds to the lowest of the stored rated lamp currents, a fluorescent tube with a low rated lamp current cannot be overloaded, even after long-term operation under these conditions. However, as the starting phase lasts only a few seconds to minutes even fluorescent tubes whose rated lamp current is lower than the dimming current will not fail.
Favourably, an optimised dimming current is set during the start phase whose current intensity is sufficient for the operation of a fluorescent tube whose rated lamp current is greater than the optmised dimming current, and which does not destroy a fluorescent tube whose rated lamp current is lower than the optimised dimming current.
For lamp types with rated lamp currents higher than the optimised dimming current, the optimised dimming current supplies enough energy to generate sufficient luminous intensity. A dimmed setting lasting a few seconds to several minutes is acceptable, as sufficient brightness is already achieved.
For simplicity, the lowest preheating current stored in the register is set at the start of the first stage of the preheating phase, which is divided into several stages. After the first stage of the preheating phase, a first YES/NO query checks whether the electrode resistance falls within one of the predefined electrode resistance ranges. In the event of a YES decision, a further stage of the preheating phase as triggered, during which the preheating current of the previous stage is retained and the starting phase subsequently initiated. A NO decision triggers a further stage of the preheating phase where the next higher preheating current stored in the register is set at the beginning of this stage. After a predefined time, either the starting phase is initiated or a further YES/NO query is performed, followed by the same process steps as after the first YES/NO query.
For simplicity, the number of YES/NO queries is predetermined and can be defined for the required ballast either inherently or by means of a data processing program. Three steps are of importance for the course of the process: every YES decision triggers a further stage of the preheating phase, where the preheating current of the previous preheating stage is retained. Every NO decision triggers a further stage of the preheating phase with the next higher preheating current. If the decision in the last YES/NO query provided for in the course of the process is NO, this triggers an increase in the preheating current, which is followed, after a predefined period, by the starting phase, without a renewed YES/NO query being performed.
A further improvement in the method is achieved by dividing the stored operating data of the recognisable lamp types into lamp groups in the register, where each lamp group contains only fluorescent tubes with different rated lamp voltages, one of the electrode resistance ranges and a preheating current are allocated to each lamp group by the register, the lamp group to which the fluorescent tube belongs is determined via the measured electrode resistance or the last preheating current set during the preheating phase, the rated lamp voltage that comes closest to the measured lamp voltage of the fluorescent tube is searched within the established lamp group of the register during the subsequent starting phase, and the operating data are then set that are necessary for operation of the fluorescent tube and allocated to the measured lamp voltage by the register. Among the commercially available fluorescent tubes, there are ones that have the same rated lamp voltage, but different electrodes and electrode resistances. In the register according to the invention, however, these are allocated to different lamp groups, meaning that unequivocal allocation of the lamp type is possible within a lamp group on the basis of the rated lamp voltage measured.
In the further development of the method, an item of information obtained during the preheating phase, i.e. the electrode resistance determined or the last preheating current set during the preheating phase, is analysed for the starting phase and the further search for the exact lamp type is restricted to one lamp group by the register.
Moreover, the starting phase can also be improved by combining the recognisable lamp types into lamp groups. For this purpose, a dimming current is allocated to each lamp group in the register, where the dimming current to be set for the starting phase is already defined during the preheating phase by establishing the lamp group.
In an advantageous further development, the procedure of the method provides for a three-stage preheating phase with two possible YES/NO queries, where a NO decision in response to the second YES/NO query triggers the third stage of the preheating phase where, compared to the previous stage of the preheating phase, the highest preheating current stored in the register is set and the starting phase is initiated after a predefined time.
The three-stage procedure offers three predefined preheating currents which, starting with the lowest, can be increased in successive stages of the preheating phase.
The three-stage design of :he method for preheating is an advantageous compromise, as it permits sufficiently differentiated preheating of the large number of lamp types and the design effort for the ballast required remains within reasonable limits.
In order to avoid damage to the ballast required, particularly if the fluorescent tube is defective and an abnormal operating condition arises where the lamp voltage rises, a maximum lamp voltage can be stored in the register for each lamp type. During operation of the fluorescent tube, a check is then made of whether the lamp voltage currently present during operation exceeds the maximum lamp voltage. If the maximum lamp voltage is exceeded, a safety shutdown of the fluorescent tube is performed. The lamp voltage check can, for example, be performed continuously or at defined intervals.
Similarly, a minimum lamp voltage can also be stored in the register and checked to ascertain whether the lamp voltage present is below the minimum value. Again, the fluorescent tube is shut down if the value is below the minimum.
The stored maximum lamp voltage is expediently greater than the highest lamp voltages stored in the register. Alternatively, different maximum lamp voltages can be stored for every single lamp type or groups of lamps types.
The starting phase is performed by operating an ON/OFF switch allocated to the lamp or, expediently, also initiated by inserting a fluorescent tube in an empty lamp socket while the lamp is switched on. This prevents a fluorescent tube being operated with the wrong operating data after being put into operation while the lamp is switched on. The same applies to the preheating phase. This can also be initiated by operating an ON/OFF switch or by inserting a fluorescent tube in an empty lamp socket.
The invention furthermore consists in a particularly simple design of a ballast for implementation of the method according to the invention, with a frequency generator and a control circuit interacting with this, which supplies the fluorescent tube with an alternating voltage via power transistors, where the lamp current can be set by a limiter, a register in which the operating data of several lamp types are stored, a sequence control system for controlling the timing of the process steps to be executed during a starting phase of the fluorescent lamp, a measured-value analyser, a lamp voltage measuring device and a direct-voltage generator with which a logic voltage can be generated.
The lamp current can, for example, be set indirectly via the frequency of the alternative voltage, by varying the direct voltage or by impedances of variable value.
The structured design of a ballast of this kind is advantageous. Its design makes it possible to supply only the control circuit and the downstream power transistors with high energy through the direct-voltage generator in order to operate the fluorescent tube.
In order to be able to perform an optimum hot start of the fluorescent tube, the ballast is provided with an electrode resistance measuring device. In addition, the sequence control system can be used to control the timing of the process steps to be executed during a preheating phase of the fluorescent tube.
The sequence control system, the measured-value analyser, the register and the frequency generator are expediently located in a common control device, this also being referred to as the controller.
The direct-voltage generator has a connection which supplies energy to the parts of the ballast involved that are involved in data processing. The energy is tapped in the form of a stabilised logic voltage, which is substantially lower than the lamp voltage required for supplying the lamp.
The control device, the control circuit, the lamp voltage measuring device and the electrode resistance measuring device are supplied with a stabilised direct voltage via the direct-voltage generator. This is tapped as a so-called logic voltage at a separate connection of the direct-voltage generator and is substantially lower than the lamp voltage required for supplying the lamp.